X-ray fluorescent screen



Sept. 8, 1953 R. LONGINI ETAL 2,651,584

X-RAY FLUORESCENT SCREEN Filed Sept. 25, 1948 Cesium Iodide Between Zinc Sulphide Crystals WITNESSES: INVENTO R$ Richard L. Longml a Walter J. Hushley.

Patented Sept. 8, 1953 X-RAY FLUORESCENT SCREEN I Richard L. Longini and Walter J; Hushley, Pitts burgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corpora- I tion of Pennsylvania Application September 25, 1948, Serial No. 51,258

Our invention relates to materials which become fluorescent to produce visible light under the impactof X-rays and, in particular, relates to a method of combining different substances to produce a maximum yield of visible radiation for .a given X-ray energization. I

' In the medical and other X-ray arts, it isfrequently desirable to make visible the X-raylpatterns produced by irradiating various objects which are opaque to'visible radiation by streams of X-rays, and screens covered by fluorescent material on which the transmitted X-ray pattern is incident have been used for this purpose However, difiiculties 'have been found in that the brightness of the image produced on the fluorescent screen is undesirably low, so that it haslbeen necessary to view the screens in darkened rooms, and to delay observation until the eyes of the 3 Claims. (CL 117-335) observer have had time to accommodate themselves to the low luminous intensity of the view being observed. 7

One object of our invention is, accordingly, to provide a fluorescent material in which the luminous intensity of the fluorescent image for a given intensity of X-ray radiation is increased over that possible in screens of the prior art.

Another object of our invention is to produce a material which produces light of a highly desirable character and intensity when irradiated with X-rays.

Still another object of our invention is to provide a novel method of combining an ancillary substance with fluorescent materials of the prior art to produce a maximum yield of radiation for a given intensity of X-radiation.

Other objects of our invention will become apparent upon reading the following description, taken in connection with the drawing in which the single figure is a sectional view of a screen provided with a fluorescent coating in accordance with the principles of our invention.

Among the materials which have been used in the production of fluorescence under the action of X-rays are calcium tungstate, zinc sulphide, and zinc-cadmium-sulphide. Other materials of somewhat analogous chemical classification are known, the name X-ray phosphors being frequently applied to materials for such purposes.

In making measurements of the luminous elliciency of such materials, we have found that r while zinc sulphide produces a much larger yield of visible radiation than does calcium tungstate for a given absorption of X-ray energy, the luminous intensity of the zinc sulphide screens is made undesirably low because zinc sulphide has a very low absorption coefiicient. We have, however, found that this difiiculty can be corrected .by admixing with the zinc sulphide an ancillary material or flux comprising elements having high atomic weights. Among the substances vparticularly useful for this purpose are alkali halides such as caesium iodide (CsI), caesium bromide (CsBr), rubidium iodide (RbI), caesium chloride (CsCl), potassium iodide (KI), sodium iodide (Nail), rubidium bromide (RbBr), and rubidium chloride (RbCl). Such materials have a strong tendency to absorb X-rays and appear'to be effective by reason of their containing an element of high atomic weight. ,These materials increase the X-ray absorption coefilcient of the mixture by amounts having, the same order in which we have just enumerated them, the caesium iodide producing the greatest relative absorption. A given one of these addition agents appears to increase'the relative absorption by an amount linearly proportional to the mole fraction of the addition agent,

It has also been found that such addition agents likewise increase the luminous efliciency of the fluorescent material, provided they are not added in too great amounts; that is to say, they increase the ratio of the energy emitted as visible radiation to the energy of the X-radiation absorbed. As is pointed out in R. L. Longini Patent 2,571,905 issued October 16, 1951 for Zinc Sulphide X-ray Phosphors, there is found to be a maximum luminous efficiency for a certain mole fraction of the addition agent, the efficiency first increasing as small amounts of the addition agent are incorporated with the pure zinc sulphide, then rising to a maximum, and then decreasing ultimately to a lower value than that of the pure zinc sulphide at large molecular fractions of the addition agent.

The proportioning of the addition agents to obtain a high luminous effioiency in the fluorescent screen is more fully described and claimed in the application just mentioned.

While the insertion of the addition agent by simple admixture with the phosphor crystals will yield the improved results described in the above mentioned Longini application, We have discovered that their addition by the processes about to be described herein produces fluorescent screens of superior form. A simple admixture or deposition of the addition agent tends to increase the thickness of the screen and while this would increase the X-ray energy absorption, the transparency and resolving power of the screen would be affected deleteriously. Therefore, in accordance with our present invention, we introduce the addition material containing the elements of high atomic weight into the empty spaces between phosphor crystals. This can be done without substantially increasing the thickness of the screen and a superior optical agency is thus attained. Thezinc sulphide in such cases is reduced, by methods too well known in the art to give description here, to the form of small discrete crystals and these are deposited on a glass or other supporting surface 2 to form a layer I, as shown in the drawing, to produce the desired screen. The addition material to be introduced into the spaces between the crys tals may then be dissolved in some suitable evaporable solvent and caused to flow over the surface of the screen and penetrate into the spaces between the crystals. Many desirable'addition agents, of which caesium iodide may be given as an example, are soluble in water and water solutions of these addition agents may therefore be used to impregnate the surface of the screen. The water or the solvent is then evaporated from the screen leaving a deposit 3 of the addition material in thespaces between the crystals.

To give another example of the method of forming screens in accordance with our invention, the individual phosphor crystals may be shaken up with powdered form of the addition agent or otherwise coated with such a powder and then deposited to form the screen.

To take still another example the addition agent may be incorporated into a suitable binder such as a potassium silicate solution with which the phosphor crystals are admixed and then deposited to form the screen.

As a specific example of our method of forming fluorescent screens one particularly adapted for employment with 80 kilovolt X-rays and having a coating of such single sulphide crystals to form a screen having 100 milligrams of zinc sulphide on each square centimeter may be impregnated with caesium iodide in the amount of 20 milligrams for each square centimeter of the screen with the result of attaining a very substantial increase in a light output per square centimeter of screen surface.

We claim as our invention:

1. An optical screen comprising zinc sulphide crystals having caesium iodide in the space intervening between them and means for supporting said crystal in a layer, the relative Weights of zinc sulphide and caesium iodide being in the ratio to 20.

2. An optical screen comprising zinc sulphide crystals having in the spaces intervening between them a material drawn from the group which consists of caesium iodide, caesium bromide, rubidium iodide, caesium chloride, potassium iodide, sodium iodide, rubidium bromide and rubidium chloride, and means for supporting said crystals in a layer, the relative weights of zinc sulphide and said material being in the ratio of 100 to 20.

3. A fluorescent screen comprising phosphor crystals coated with a substance drawn from the group consisting of caesium iodide, caesium bromide, rubidium iodide, caesium chloride, potassium iodide, sodium iodide, rubidium bromide, and rubidium chloride, and means for supporting said crystals in a layer, the relative weights of said phosphor and said substance being in the ratio of 100 to 20.

RICHARD L. LONGINI. WALTER J. HUSHLEY.

References Cited in the file of this patent UNI'I'ED STATES PATENTS Number Name Date 2,243,828 Leverenz May 2'7, 1941 2,310,740 Leavy Feb. 9, 1943 2,435,435 Fonda Feb. 3, 1948 2,435,436 Fonda Feb. 3, 1948' FOREIGN PATENTS Number Country Date 449,392 Great Britain June 19, 1936 

1. AN OPTICAL SCREEN COMPRISING ZINC SULPHIDE CRYSTALS HAVING CAESIUM IODIDE IN THE SPACE INTERVENING BETWEEN THEM AND MEANS FOR SUPPORTING SAID CRYSTAL IN A LAYER, THE RELATIVE WEIGHTS OF 